transaction: simplify `undo.backupfiles` file creation with a variable This make the code shorter and clearer. (This is an opportunity improvement while looking at something next to that.) Differential Revision: https://phab.mercurial-scm.org/D10846

transaction: narrow the error filtering when failing to rename undo file Having inconsistent "undo" files can be quite serious so we narrow the error filtering to the intent that the comment explain. (This is an opportunity improvement while looking at something next to that.) Differential Revision: https://phab.mercurial-scm.org/D10845

transaction: clarify a conditionnal about version check Let us move the short branch early. Differential Revision: https://phab.mercurial-scm.org/D10844

transaction: extract message about different version in a constants This make the code a bit simpler and clearer. (This is an opportunity improvement while looking at something next to that.) Differential Revision: https://phab.mercurial-scm.org/D10843

transaction: explain why some recovery failed Right now we issue a message about failing to recover some file, but not why. It seems useful to add some information about that. (This is an opportunity improvement while looking at something next to that.) Differential Revision: https://phab.mercurial-scm.org/D10842

copyfiles: deal with existing file when hardlinking If the hardlinking fails, we fallback to `shutil.copy`, but do not consider future hardlinking doomed. This is a slight improvement from the current situation, we still avoid hardliking in a case we might be able to do it. However this does not have an impact of the rest of the operation. (This is an opportunity improvement while looking at something next to that.) Differential Revision: https://phab.mercurial-scm.org/D10841